This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. This is an open-label single-dose study conducted in 3 to 10 centers in four groups of pediatric patients with ASD, ages 5 to 16 years (inclusive), stratified by weight. A total of 24 patients will be enrolled (6 patients per group). Patients will be assigned to one of the following groups: Group A: 60 to 80 kg Group B: 40 to 59 kg Group C: 20 to 39 kg Group D: less than 20 kg Enrollment will proceed sequentially by group, with Group A patients enrolling first and Group D patients enrolling last. Following enrollment of Group A, each subsequent group of patients will be enrolled after 1) the pharmacokinetics of memantine have been evaluated for at least two patients in the preceding group and 2) drug exposure in terms of AUC for patients in the preceding group has been determined to be less than a predefined limit that is deemed to be safe. All patients will receive a single 3-mg dose of a memantine HCl MR capsule in the morning of Day 1. The total duration of each patient's participation in the study will be 8 days (Day 1 through the last PK sample collection on Day 8). Safety will be assessed throughout the study by monitoring AEs, physical examinations, ECGs, vital sign assessments, and clinical laboratory tests. The purpose of this study is to evaluate the pharmacokinetics of a single, low, oral dose of memantine HCl modified release formulation in four groups of pediatric patients with autism stratified by body weight.